High-performance carbon fibers can be classified into PAN-based carbon fibers obtained from polyacrylonitrile (PAN) and pitch-based carbon fibers obtained from pitches. Carbon fibers are widely used in aerospace, construction and civil engineering, and sport and leisure applications, making use of their feature that they have much higher strength and elastic modulus than ordinary synthetic polymers.
While much attention is now paid to methods for making efficient use of energy, typified by energy saving, the generation of Joule heat from high-speed CPU's and electronic circuits is becoming an issue. To solve these, the efficient processing of heat, so-called “thermal management” must be attained.
Although carbon fibers have a higher thermal conductivity than ordinary synthetic polymers in general, the further improvement of thermal conductivity is now under study. Commercially available PAN-based carbon fibers generally have a thermal conductivity lower than 200 W/(m·K). In contrast to this, it is perceived that pitch-based carbon fibers easily attain a higher thermal conductivity than PAN-based carbon fibers.
However, to make carbon fibers actually function as a thermal conductive material, the thermal conductivity of a molded product of the carbon fibers must be improved. To make thermal conduction equal three-dimensionally, a filler which mainly serves to conduct heat must form a three-dimensional network. For example, in the case of a spherical filler which is uniform in size, the network of the filler contained in a molded product behaves like percolation when it is dispersed uniformly though this depends on the dispersion state of the filler. Therefore, in order to obtain a sufficiently high thermal conductivity and electric conductivity, a certain amount or more of a filler must be added. However, it is extremely difficult to disperse a medium and a filler in a concentration higher than a certain level in most cases when a molded product is to be formed. Although a composite material composed of a fabric formed of conventionally used fibers and a matrix has an improved thermal conductivity in the in-plane direction, it is hard to say that its thermal conductivity in the thickness direction is satisfactory because carbon fibers cannot form a network completely.
Under the above situation, many attempts have been made to improve the thermal conductivity drastically. JP-A 5-17593 discloses a thermal conductive molded article having high mechanical strength which is manufactured by impregnating carbon fibers drawn in one direction with graphite powders and a thermosetting resin. JP-A 2-242919 discloses that physical properties such as thermal conductivity of a molded product are improved by the improvement of the physical properties of carbon fibers. However, the improvement of the thermal properties of a molded product is not clear therefrom.
Development is under way from the viewpoint of increasing the thermal conductivity of carbon fibers as described above. However, from the viewpoint of thermal management, the thermal conductivity of a molded product must be high. Then, it is desired that carbon short fibers having a suitable thermal conductivity can be controlled to show optimum thermal conduction in a molded product, or a carbon fiber reinforcement which has a suitable thermal conductivity and can increase the content of carbon fibers in a molded product and a carbon fiber reinforced composite material which has an improved 3-D thermal conductivity and excellent mechanical properties are strongly desired.